Our long term goal is to develop and apply computational methods to provide novel mechanistic insights into catalysis and regulation of histone modifying enzymes, and to facilitate the rational design of enzyme sub-type specific modulators for probing epigenetic pathways and therapeutic use. Reversible histone acetylation has emerged as a vital regulator in a multitude of essential epigenetic processes. The enzymes responsible for this essential post-translational modification are histone acetyl transferases (HATs) and histone deacetylases (HDACs) that add and remove acetyl groups to and from target lysine residues, respectively. The aberrant activity of these enzymes has been implicated in numerous human diseases, notably cancer, and quite a few HATs and HDACs have been established as important drug targets. Our theoretical approaches will center on Born-Oppenheimer ab initio QM/MM molecular dynamics simulations, a state-of-the-art computational approach to simulate enzyme reactions which allow for accurate modeling of the chemistry at the enzyme active site while properly including dynamics and effects of protein environment. The specific aims are: 1. Characterize the catalytic mechanism for HATs and rational redesign of tGcn5 for its improved efficiency. Aim 2: Elucidate inner workings of sirtuins, a novel family o class III histone deacetylases. Aim 3: Advance ab initio QM/MM methods. The successful completion of the proposed research will provide a detailed mechanistic understanding for HATs and sirtuins for the first time. This will stimulate further mechanistic studies of these important enzymes, and facilitate the development of novel mechanism-based modulators for probing acetylation dependent epigenetic pathways and for therapeutic use. Meanwhile, our methodology development efforts will significantly advance this computational tour de force to simulate enzyme reactions, and help establish it as an equal partner to experimental approaches in this important field of enzymology.